Legend has it that there are an infinite number of Sean Beans on that farm, grazing together, happily.

This is just the cast list for the open casting. You'll note Arianne isn't on here either.

Pounce comes back as the actor who portrayed Benjen season 1

Or Summer and Ser Pounce realize how much they resemble each other and switch places, unbeknowst to all the other characters. "The Pounce and the Pupper"

He turned down lucrative film deals multiple times, though. The only reason he accepted HBO in 2006 was because he thought they'd do a good job.

Third unlikely but present possibility - they recently had a stillborn girl/had a third daughter who died very young. I don't actually believe this, but it would fit if they were really stretching for a red herring.

I've always liked the theory that her name is also Robin, hence why the kids are confused when the girl in the bar named Robin turns out not to be their mom. Also, it would be fun to see Marshall and Lily debate whether Ted ended up with Robin or not for the purpose of their bet. That said, it might be a little…